Diabetes mellitus is a widespread global epidemic threatening human health. Blood glucose self-monitoring is essential to prevent complications of diabetes in assessing normal glucose levels. Changes in diet, exercise and medications alters blood glucose in unpredictable ways, often requiring frequent skin punctures for traditional assessment of glucose levels. Effective glycemic monitoring and control is required in avoiding acute and chronic complications such as diabetic coma, protein glycation, crosslinking, microvascular disease and large vessel disease. A continuous non-invasive measure of blood glucose and glycated hemoglobin would provide diabetic patients with the information required to monitor and control the condition.
As shown in FIG. 1, glycated hemoglobin (HbA1c) may be an indicator of diabetes risk. It has been found to be a better indicator than the commonly used measure of fasting glucose. Recent research by the Johns Hopkins Bloomberg School of Public Health has suggested that measurements of HbA1c more accurately identify individuals at risk of diabetes than the commonly used measurement of fasting glucose. The HbA1c test has low variability on a daily basis and levels are not as affected by illness and stress. It is a more stable marker of mean blood glucose over three months and as such the patient does not need to fast prior to the test being performed.
Optical devices such as near-infrared spectroscopy (NIRS) for the measurement of blood characteristics have been used in many areas of blood and tissue constituent diagnosis, cholesterol levels, liver enzymes (aspartate transaminase and alanine transaminase), bilirubin levels, lactic acid levels, blood oxygen saturation, glucose, hemoglobin, glycated hemoglobin and others. Different blood constituents have varying optical properties depending on conjugation, chirality, molecular weight and electron cloud topology. These optical properties can be used to identify the IR spectral signature of soft tissue absorption, transmittance and scattering in a blood constituent of interest. An example of this is shown in FIG. 10 where a comparison of IR absorption spectra is shown for glycated (HbA1c) and normal hemoglobin (Hb). Various blood constituents require the use of specialized light frequencies to maximize absorption in order to detect light intensity which often directly correlates to concentration and osmolarity. These specialized light frequencies require the use of narrow band infrared light sources such as infrared light emitting diodes (IR LEDs), or index and gain guided lasers and its compliment detector pairs (IR sensitive photo transistor).
A particularly well-known technique for the measurement of blood characteristics is pulse oximetry. A pulse oximetry device, such as shown in FIG. 2, measures the oxygen saturation of blood. Pulse oximetry involves the transmission of two or more wavelengths of infrared (IR) light where blood perfuses the tissue, for example at a finger or earlobe. An IR LED is the most common source of the light used in pulse oximetry. A photodetector, such as a photodiode or a photo transistor, senses the absorption and transmittance of light from the other side of the tissue.
Simple phototransistors can only sense absorption and transmittance of light. To enhance IR spectral content and its intensity spatial properties, integrated mobile cameras and LED flashes can be used to extract blood absorption, scattering and transmittance IR spectra intensity matrix data.